This invention relates to the control of fluid flow through a conduit and, in particular, relates to the control of hot gas through a relatively large conduit.
A combined cycle power plant is one in which a gas turbine and a steam turbine are combined to produce electricity. The connecting link between the two is a heat recovery steam generator (HRSG) wherein there is a heat transfer relationship between the hot gas turbine exhaust gas and the steam turbine feedwater which is converted to steam. The exhaust end of the gas turbine is connected by a conduit with the HRSG and gas flow between the two is controlled by a so called isolation damper. There are times when it is desirable to isolate the HRSG from the hot exhaust gas and for this purpose there is provided a bypass stack which will exhaust the hot gas to the atmosphere. Gas flow through the bypass stack is controlled by a bypass damper. This arrangement is clearly shown in U.S. Pat. No. 3,934,553 for a "Combined Wall Burner and Flame Holder for HRSG" and assigned to the assignee of the present invention.
Two basic and related problems associated with HRSG dampers are addressed in this application. One problem is warping and other forms of thermal distortion which may be found in both the frame and damper blade. Another related and corollary problem is leakage through the damper. In the prior art it has been found necessary to reinforce damper blades with heavy rib struts to provide for stiffness of the blade. The damper blade is a relatively thin metal sheet whereas the ribs are relatively heavy leading to unequal relative thermal expansions. Another problem is that only one side of the damper blade is heated when the damper is closed. Unequal heating and unequal masses can result in thermal deformation and warp which substantially increases gas leakage and can lead to damper blade cracking so that replacement or repair is necessary.
The frame which contains the damper blade is also subject to thermal deformation which may cause the frame sides to bow inwardly resulting in an interference with the blade and/or cause the structure to fail at corner joints or to twist. Differential expansion between the frame and adjacent ductwork can lead to distress and leakage at the frame flanges.
In designing the damper structure for near zero flow; i.e., a complete blocking of flow when closed it is desirable to provide some margin for slight thermal deformation. This is provided in the present invention as will be more particularly pointed out in the following paragraphs.